Liquid protection of electrodes



Nov. 28, 1967 MOORE, JR 3,354,644

LIQUID PROTECTION OF ELECTRODES Filed June a, 1965 United States Patent()fiice 3,354,644 Patented Nov. 28, 1967 3,354,644 LIQUID PROTECTION OFELECTRQDES Robert David Moore, In, Los Angeles, Calif, assignor toElectro-Optical Systems, Inc., Pasadena, Calif, a corporation ofCalifornia Filed June 8, 1965, Ser. No. 462,202 11 Claims. (Cl. 60202)This invention relates to a protective coating for a member subject toerosion by particle bombardment, vaporization, as well as other sourcesof erosion. In particular, this invention relates to electrodes,contacts and the like wherein a replenishable liquid is distributed overthe surface of such members by surface tension forces and capillaryaction, whereby the surface of the member is protected.

It is well known in the electrical art that the erosion anddeterioration of electrodes severely limits the life of such elementscreating performance, maintenance and reliability problems along withincreasing costs. To prevent this erosion, various prior art approacheshave been taken, such as employing erosion resistant materials,minimizing the erosive forces and cooling the electrode by internalcooling arrangements. In some applications, these techniques have beenproven helpful but in general it has meant only a slight lifetimeincrease. In many devices these techniques have not been applicable.These prior art techniques have had little success in the case ofelectronic device electrodes where the electrode controls the flow ofelectrons or ions. An ion engine and various electronic tubes aretypical devices employing such electrodes. In these devices theelectrodes are subject to ion and electron bombardment which tends toerode their surface decisively limiting their life and deterioratingtheir geometry. In such electrodes it is not possible to minimize thecause of erosion, as such is incident to normal operation of thedevices. It is not practical to conventionally cool such electrodes,especially in the case of applications where weight and cost are animportant factor. The application of improved materials can at best onlyslightly extend the life of such electrodes.

In order to overcome the shortcomings and disadvantages of prior artdevices, a simple means for automatically and continually covering thesurface of an electrode with a readily replenishable material has beeninvented. In particular, a liquid is distributed over the surface of theelectrode by surface tension and capillary forces. These forces may bevisualized as similar to the ones which occur when an ink blotter istouched to a wet wick. The

blotter is immediately wet over a substantial area. In the invention amember containing a liquid and having relatively large pores is touchedwith a member having relatively small pores resulting in a wetting ofthe member aving the relatively small pores. The liquid material andelectrode material are such that the electrode is wetted or coated bythe liquid. As the electrode is bombarded with particles, the liquid iseroded and the continual capillary and surface tension forcesautomatically replenish the liquid over the surface of the electrode.

By employing this invention, it has been found that indefinitely longlifetimes may be achieved. In early tests of the invention, lifetimeimprovement of fifteen to one (15:1) under the most severe conditionshas been achieved. The integrity of the electrode geometry is maintainedthroughout such electrode life. This is especially important in deviceswhere the electrode shape controls particle flow or movement oralternately controls the shape of the part being formed, such as inelectric discharge ma-chining and forming. In addition, the inventedarrangement is relatively simple, lightweight, may increase reflectionfrom the electrode to a radiating body where desired, enable operationunder most severe conditions, reduce electron emission from theelectrode, and impart other desirable properties to the electrode.

Briefly, the structure of the invention comprises: a porous electrodemember for exerting an electric field, a reservoir means for supplying aliquid metal to said porous electrode member, and a liquid metal in saidreservoir, whereby said electrode is covered with said liquid metal.

The above advantages and structural features will be readily understoodby referring to the detail specification which follows along with thedrawings wherein:

FIGURE 1 is a schematic drawing of an ion engine employing the inventedelectrode;

FIGURE 2 is a front view of an electrode embodying the invention;

FIGURE 3 is a sectional elevation view taken along the lines 3-3 ofFIGURE 2; and,

FIGURE 4 is an enlarged view of a portion of FIG- URE 3.

This invention will be explained in conjunction with an ion enginebecause it is in this application that the invention is exposed tosevere operating conditions. It will be apparent from this particularuse that the invented member may be readily employed in many lessrigorous applications. Referring to FIGURE 1, an ion engine is showncomprising an ionizing material reservoir 10 which may contain anionizing material 12, such as cesium. The ionizing material is passedthrough a vaporizer 14 which is kept at the boiling point of theionizing material which is usually stored in reservoir 10 in liquidform. Vaporized ionizing material is passed from vaporizer 14 via thefeed line 16 to the back side of an ionizer 18. The ionizing materialthen passes through the ionizer 18 which may be a porous tungsten ionemitter that is maintained at a temperature in the negihborhood of 1200C. The ions emitted from ionizer 18 are accelerated by an acceleratingelectrode 20 which is connected to an energy source 22. A thermionicneutralizer 24 is inserted in the path of the ions to prevent the ionsfrom being attracted to the space vehicle after being ejected from theengine. It is the attraction and acceleration by the acceleratingelectrode Ztl and the ejection of the ions from the engine that producesthe desired engine thrust. This general description of an ion engine isonly for purposes of placing the invented electrode in an environmentwhich will clearly point out its significance and advantages. Theadvantages are present when the invention is employed in many differentenvironments and this one should only be considered illustrative. Thesignificance and advantages of the invention can be appreciated byconsidering FIGURES 24.

Referring to FIGURES 2-4, the electrode 20 comprises an electrode member26 that is subject to erosion by bombardment with ions, by vaporizationdue to high temperatures or by other sources. In an ion engine, theprimary source of erosion is ion bombardment. An ion engine is commonlyoperated at temperatures in the range of 800 to 900 C. and at pressuresof less than 10 torrs. Under such operating conditions, it is preferredthat the electrode be made from a sintered tungsten material having amaximum pore size of approximately 10 microns. Other materials such asiron and molybdenum may also be employed in an ion engine as electrodes.It should be understood that it is only necessary that the pore size bean appropriate size relative to the other elements of the electrode. Theelectrode material should be adapted to withstand the operatingconditions of the electrode, should be compatible with the liquidemployed in the electrode, and should have properties consistant withthe function performed by an electrode.

As shown in FIGURE 2 the electrode is circular in shape and has ahexagonal portion 28 which includes a plurality of apertures 30 whichfacilitate the passage of the ions through the electrode. Situatedadjacent each side of the hexagon 2 8 is a means for storing a liquidsuch as reservoir means 32. The reservoir 32 is attached to theelectrode member 26 and the electrode member 26' is operated at atemperature which maintains the material in reservoir 32 in its liquidstate, thus additional heat sources are not required to maintain theliquid state. A cap (not shown) or other sealing means is placed overthe reservoir to retain the liquid in reservoir 32. While there are aplurality of reservoirs 32 shown, it should be understood that it ispossible to employ fewer reservoirs. It is possible to distribute liquidover the surface of the electrode with one reservoir. The determinationof the exact size, the number and the nature of the reservoirs willdepend upon geometry, construction and the size of the electrode alongwith the period dur: ing which the reservoir 32 is not to be replenishedand the importance of minimizing Weight by notincorporating additionalheating sources (that is, other than the elec trode member 26). It is,of course, within the scope. of the invention to employ other types ofreservoirs such as a separated tank with an independent heat sourcealong with a wick connecting the independent tank and the electrodemember 26 or reservoir 32. Other reservoir and heating combinations mayreadily be devised for bringing a liquid in contact with the electrode26.

The reservoir 32 has pores that are relatively large in comparison withthe pores of the electrode member 26. In the specific embodimentdescribed above, the pores of the reservoir 32 should be larger than 10microns. These dimensions in general refer to the width or diameter ofthe pore. The term pore should not be taken as designating a particularconfiguration or shape. A pore may be any hollow or evacuated region oropening. Typically, the reservoir 32 may be made from a honeycomb orfoil or a sponge structure of iron, aluminum, tungsten, molybdenum orother materials depending on the liquid used.

An important aspect of this invention is the liquid 34 that is employedin the reservoir 32 FIGURE 4. This is especially so in the case of anion engine where the materials involved, ion bombardment and operatingconditions (temperature, pressure, etc.) present diflicultqualifications. The liquid, 34 must first be capable of wetting theelectrode surface. The liquid is said to wet a surface or solid when thecontact angle between the liquid and the solid is less than 90 andpreferably equal to This is necessary to allow surface tension andcapillary action to combine to flow a liquid through the electrode andform a film over the electrode surface by merely bringing, the fluid inreservoir 32 into contact with the smaller pores of the electrode.Stated simply, the smaller pores of electrode member 26 give rise to. agreater surface area than the surface area of reservoir 32. resulting inthe electrode member 26 exerting a greater surface tension force onliquid 34 than reservoir 32 This differ.- ential of surface tensionforces combines with capillary action to spread the. fluid over theelectrode surface by capillary action.

In addition to wetting, the electrode material must not melt or dissolveinto liquid 34 to any appreciable extent and the vapor pressure ofliquid 34 must be low enough at the electrode temperature that the massloss of' liquid 34 due to evaporation is a small fraction of the massflow of ionizing material employed in the ion engine. In any case, themass loss due to evaporation should be maintained at a relatively lowfigure in order to conserve liquid 34. In the case of an ion engineoperated in space or in regions approaching the environment of space,the vapor pressure of liquid 34 would have to be under approximately 10"torrs. Other requirements for liquid 34 are that it remain liquid at allelectrode operating temperatures, that it not react with the ionizedmaterial, that it have high enough vapor pressure to evaporate fromionizer 18 faster than it is sputtered on to it and that it either notreact with the ionizing material, or if reacting, form compounds whichwill liquify or evaporate at the electrode operating temperatures. Thislast requirement is necessary. to prevent the electrode surfaee frombecoming choked with ionizing materialr-liquid compounds which would.interfere with the liquid flow or destroy the surface tension action. Inan ion engine environment employing a sintered tungsten electrode andcesium ionizing material with the engine operating at temperatures inthe range of 800 to 900 C. and pressures of less than 10 torrs, a liquidm t of tin d 2 ron b Wei ht l a e satis' factorily.

In operation, liquid metal 34 is placed in the reservoir 32 by suitablemeans andthe ion engine electrode mem ber 26 is brought up, to operatingtemperatures in the range of 800 to 900 C. whereupon the material in:reservoir 32 retains or assumes a liquid state. With the metal 34 in itsliquid state, the difierential in pore size between electrode member 26and reservoir 32' results in the electrode member 236 soaking up a givenamount of liquid metal 34 and automatically distributing this metal overits surface by capillary and surface tension action. The liquid metal 34forms a film over the surface of the'electrode member 26. As the ions,are accelerated from ionizer 18 through electrode member 26, a number ofthem collide with electrode member 26' and act as an erosive force. Theliquid film acts as a protecting surface and though it may bemomentarily broken by the ion bombardment, the'reservoir 32 and thecapillary and surface tension action function to, soon thereafterreplenish the eroded material. This eroding and replenish ing actionserves to greatly extend the liftof' the electrode and to preserve itsgeometry. In the case of an ion engine as well as other electronicdevices, the coat ing of the electrode with a liqui'd'metal may, withproper choice of liquid, reduce the electron emission of theelew trode.In the case of an ion engine, the emission of electrons results in apower loss. The reduction of this electron emission may increase the,efficiency of an ion engine by as much as 2%. In addition, in anionengine the coating of the electrode surface adjacent ionizer 18 with afluid such as tin results in this surface of the electrode reflecting asubstantial portion ofthe radiant energy transmitted by ionizer 18. Thisboth adds to the efficiency of the engine and helps in reducingelectrode temperature. The invented arrangement is lightweight, simpleand relatively low in cost. These features, broaden its scope ofapplication.

It should readily be apparent from the above. description pertainingp-rimarily to an ion enginethat the invented arrangement employing areservoir, an electrode having different pore sizes and a liquidmetaldistributed from the reservoir to the electrode by capillary and/orsurface tension action have application in many other fields. Forexample, the inventedarrangementcould be employed to protect electrodesin other electronic devices such as microwave amplification tubesemploying plasmas or to protect the contacts in circuit breakers,vibrators and other electrical devices or to protect the electrodes inspark discharge machines, as well as. a. host of other applications.Therefore, various, embodiments of the present invention in addition towhat has. been described in. detail may be employed. without departingfrom the scope of the inventionwhich, is defined. in the claims whichfollow.

h t s. laim d 1 An electrical member having a potential applied thereto.comprising:

a porous member subject to. erosion, said porous. meme berhaving poresof a first given. volumeth-at extend at least over a part of the surfaceof said member;

a means for supplying a liquid to said pores of said porous member;

said means having pores of a volume greater than the pores of saidporous member to attract the liquid from said means to said member bysurface forces and to distribute the liquid over said surface; and,

a liquid in said means that is adapted to flow from said means to saidporous member and be distributed through said pores, whereby said porousmember is covered with said liquid and the erosive forces which saidporous member is exposed to are substantially absorbed by said liquid.

2. The structure recited in claim 1 wherein said porous member is anelectrode, said means is a metal sponge, and said liquid is a liquidmetal maintained in a liquid state by the temperature of said electrode.

3. The structure recited in claim 2 wherein said electrode includestungsten and said liquid metal includes tin and iron.

4. In an ion engine having a reservoir of ionizing material, a vaporizerfor vaporizing said ionizing material and an ionizer for ionizing saidvaporized material, the combination comprising:

an electrode in the path of said ionizing material for accelerating saidionized material;

a liquid coating the surface of said electrode; and,

means for maintaining a substantial portion of a surface of theelectrode coated with said liquid metal, whereby erosion of theelectrode is prevented.

5. In an ion engine having a reservoir of ionizing material, a vaporizerfor vaporizing said ionizing material and an ionizer for ionizing thevaporized material, the combination comprising:

an electrode of porous material having pores of a first given size;

said electrode in the path of said ionizing material for acceleratingsaid ionized material;

a liquid for coating at least a part of the surface of said electrode;and,

means having pores larger than the pores of said electrode formaintaining a substantial portion of a surface of the electrode coatedwith said liquid, said liquid flow from said means to said electroderesulting from differential surface tension forces and the capillaryaction of said electrode and said means, whereby erosion of theelectrode is prevented.

6. The structure recited in claim 5 wherein said liquid is a tin-ironalloy.

7. An electrode comprising:

a porous electrode member field;

a porous reservoir means for supplying a liquid metal to said porouselectrode member, said means having pores larger than said electrodemember; and,

a liquid in said reservoir means, whereby said electrode is covered withsaid liquid.

8. The structure recited in claim 7, wherein said liquid is an alloyincluding iron and tin.

9. An electrical member having a potential applied thereto comprising amember subject to erosion and normally operated at temperatures inexcess of 700 C., and a means for supplying a liquid over the surface ofsaid member, said liquid having a vapor pressure at temperatures inexcess of 700 C. that maintains the mass loss attributable toevaporation as a negligible amount.

10. The structure recited in claim 9, wherein said liquid is a metal andsaid member is a metal.

11. A method for preventing erosion of an electrical member comprisingoperating such member at an excess of 700 C.; and covering at least apart of said member with a liquid which has a vapor pressure thatmaintains evaporation mass loss at a negligible quantity at temperaturesin excess of 700 C.

for exerting an electric References Cited UNITED STATES PATENTS3,014,154 12/1961 Ehlers et a1 35.5 3,026,806 3/1962 Runton et a1.102-925 3,138,009 6/1964 McCreight 60-35.6 3,149,459 9/1964 Ulam 6035.53,209,193 9/1965 Sheer et a1. 313-63 CARLTON R. CROYLE, PrimaryExaminer.

4. IN AN ION ENGINE HAVING A RESERVOIR OF IONIZING MATERIAL, A VAPORIZERFOR VAPORIZING SAID IONIZING MATERIAL AND AN IONIZER FOR IONIZING SAIDVAPORISED MATERIAL, THE COMBINATION COMPRISING: AN ELECTRODE IN THE PATHOF SAID IONIZING MATERIAL FOR ACCELERATING SAID IONIZED MATERIAL; ALIQUID COATING THE SURFACE OF SAID ELECTRODE; AND, MEANS FOR MAINTAININGA SUBSTANTIAL PORTION OF A SURFACE OF THE ELECTRODE COATED WITH SAIDLIQUID METAL, WHEREBY EROSION OF THE ELECTRODE IS PREVENTED.